1. Field of the Invention
The present invention relates to the field of free piston machine control systems, and more particularly relates to a transient temperature range control system for avoiding damaging collisions within a free piston cooler when operating in the cool-down transient temperature range between start-up and steady state operation.
2. Description of the Related Art
The Stirling engine was invented in the early 1800""s but was not used as a refrigeration cycle until 1834 when John Hershel used a closed cycle Stirling engine to make ice. The basic concepts of the free piston engine and the free piston cooler were invented in the 1960""s by William T. Beale and are shown in U.S. Pat. No. Re30,176. Many advances, including those in bearing technology, low clearance seals, and regenerative materials, have vastly improved the reliability and efficiency of free piston machines.
The free piston cooler is essentially a pressure vessel enclosing a piston and displacer reciprocating in one or, more typically, separate, coaxial cylinders. The piston is driven in linear reciprocation and alternately compresses and expands a working gas to create a pressure wave as a function of time. The displacer is driven in reciprocation by the pressure wave acting upon a net pressure differential area and alternately moves or shuttles a major portion of the working gas between a cold head, where thermal energy may be extracted from a cold head environment, and a warm end, where heat is rejected to a warm end environment. The piston and displacer are phased so that the piston expands the working gas when the major part of the working gas is in the cold head and compresses the working gas when the major part of the working gas in at he warm end. Thus, heat is absorbed at the cold head during expansion of the working gas and heat is rejected at the warm end during compression of the working gas. There are, of course, various alternative drive arrangements for moving the piston and displacer in the desired reciprocation at the desired phase relation.
The piston is free because no mechanical linkage confines the piston to a fixed path of reciprocation. The free piston is typically driven in reciprocation by a linear electric motor. Typically, in order to maximize the efficient use of available drive power, free piston machines are driven at their frequency of mechanical resonance. Because the piston is unconfined in a free piston machine, the amplitude of reciprocation, also referred to as the stroke, is a function of piston drive force and varies under the influence of changing operating conditions. Consequently, the piston, as well as any other reciprocating structures, can collide at either end of the piston stroke with physical structures at the end of the cylinder.
In particular, in such freely reciprocating machines the amplitude and frequency of reciprocation are a function of inertia, damping, and spring and driving forces. Therefore, these machines share the common feature that, when they are overdriven or underdamped, the reciprocating parts can acquire an amplitude of reciprocation that exceeds the internal geometric limits of the space available for the motion of the reciprocating parts. If the amplitude of reciprocation is allowed to increase beyond these limits, the reciprocating parts will collide repeatedly with stationary structures, or even with other reciprocating parts. Such collisions are obviously undesirable because they may damage the machine.
Of particular concern, with respect to the present invention, is the initial, transient cool-down period when the Stirling cooler first begins operation. The period of time from the start of operation of a free piston cooler until the cold head reaches a desired set point temperature is the transient cool-down period. The initial temperature of the cold head during the transient cool-down period is often room temperature, approximately 300xc2x0 K, and the desired set point temperature at which the cooler will eventually operate is often extremely cold, perhaps as low as approximately 77xc2x0 K, or colder. As the working gas transitions from the warm initial cold head temperature to the cold set point cold head temperature, the properties of the working gas change greatly, thereby affecting the machine operating dynamics during the transient cool-down period. Specifically, as the working fluid cools it becomes more dense and viscous and, therefore, the damping of the reciprocating displacer and piston increases accordingly as the operating temperature decreases. For example, the density and viscosity of the working fluid may increase by a factor of 4.
This density change creates a problem at start up during the transient cool-down period because the control system is typically a feedback control system designed to control piston stroke under normal operating conditions, i.e. at the lower operating temperature, where damping from the working gas is greater. Consequently, at start up the cooling demand is maximum, the control system tends to drive the piston at the drive force and power which would be appropriate for the lower operating temperature but the damping is less during start up than at the operating temperature for which the control system was designed. Therefore, unless other provision is made, the piston will be overdriven at the warmer, temperatures where damping is less and collisions can result.
One prior art solution is to under power the piston drive during the transient cool-down period to assure that collisions do not occur. The piston drive amplitude is gradually or incrementally increased, sometimes manually, until, over a sufficient time period, the operating temperature is reached.
However, the problem with prior art solutions is that, not only is it desirable to avoid such collisions during the transient cool-down period, but also it is desirable to reach the desired operating temperature as quickly and efficiently as possible. Therefore, for all the intermediate temperatures throughout the entire transient cool-down period it is desirable to operate at the maximum stroke that will not result in collisions in order to pump heat from the cold head at the maximum rate of heat transfer and thereby bring the cooler to its steady state operating temperature as soon as possible.
Numerous prior art control systems have been developed to prevent collisions from occurring within free piston machines after they reach their normal operating temperature. The driving force or power applied to the piston to force it in its maximum reciprocating linear oscillation is initially much less than that required when the colder set point operating temperature is reached, in part because of the increase of the working gas density as the machine cools down. Accordingly, conventional feedback control systems designed to prevent collisions when operating at steady state temperatures, i.e. not in the transient cool-down period, would allow too much driving force to be applied to the piston during the transient cool-down period, thereby promoting collisions during that period. Such control systems have failed to address the unique conditions presented upon start-up of the machine. Such prior art control systems are generally only effective once the machine has reached steady state temperature operation.
To accomplish a maximization of stroke over the temperature range as the temperature of the cold head decreases, the drive must be progressively increased as the temperature is reduced and therefore, the limit must be progressively increased. A difficulty of doing this in an automated control system arises because there is no known algorithm or relationship applicable to each machine for relating the cold head temperature to maximum piston drive power.
For purposes of describing the invention, the term xe2x80x9ccylinder end structurexe2x80x9d is used to refer to a physical body at either end of the linear path of piston reciprocation with which the piston, or structures linked to and oscillating with the piston, can collide if the piston""s amplitude or oscillation increases excessively. The term xe2x80x9cpiston drivexe2x80x9d or xe2x80x9cdrivexe2x80x9d is the driving force or power applied to the piston to force it in its reciprocating, linear oscillation. Since piston amplitude is an increasing function of piston drive, an increase or decrease in piston drive, respectively increases or decreases the amplitude of piston oscillation if other parameters remain constant or undergo only variations which do not completely negate the change in piston drive.
Therefore, in summary, workers in the free piston Stirling machine industry have recognized the need for including, either within the control system or as auxiliary structures, a means for preventing free piston machines from damaging or destroying themselves by internal collisions. Prior art free piston machine control systems have focused on the steady state operation of the machine. During this steady state operation, if an initial collision is not recognized and the control system does not make the proper corrections, the magnitude of the collisions can increase with each cycle, often leading to irreparable damage. Such conventional control systems have not attempted to maximize piston stroke, and thereby minimize the time until the machine reaches steady state operating conditions, during the transient cool-down period and at the transient temperature conditions which occur after a free piston machine begins operation and while it works toward steady state operation. Control systems adapted to handle the unique conditions experienced in the transient temperature range are particularly important for free piston coolers.
It is, therefore, an object and feature of the present invention to provide a control system for a free piston cooler, which, under all operating conditions including transient start up conditions, maximizes piston stroke and therefore minimizes the transient time period, while avoiding collision of the piston, or component structures reciprocating with the piston, against cylinder end structures.
In one of the many preferable configurations, the transient temperature control system and method of the present invention controls a free piston cooler, having a cold head and a warm end and includes a cold head temperature sensor, a relational interface, and a temperature controller. The cold head temperature sensor senses the temperature of the cold head and generates a temperature signal. The relational interface is in communication with the temperature signal and contains a predetermined relationship between the cold head temperature and a maximum piston stroke during the transient cool-down temperature range. The relational interface generates a transient range maximum allowable stroke signal from the temperature signal and the predetermined relationship. The temperature controller preferably also controls the stroke of the piston while the cold head operates at a steady state cold head temperature. The temperature controller is in communication with the relational interface and is capable of receiving the transient range maximum allowable stroke signal and using it in limiting the piston stroke to prevent collisions within the cooler during the transient cool-down temperature range.
The relational interface contains the predetermined relationship between the cold head temperature and the maximum piston stroke during the transient cool-down temperature range. The predetermined relationship may be determined via computer modeling of the free piston cooler or through experimentation. The relationship may be established by operating the free piston cooler during the transient cool-down temperature range and recording the stroke that results in a collision, referred to as a collision stroke, between cooler components. A stroke reduction factor is then applied to the collision stroke, to provide a safety margin, to produce a transient controlled stroke used in generating the transient range maximum allowable stroke signal. The predetermined relationship is often effectively stored as a plurality of data, which may be expressed in tabular form, or as an algorithm.